Method of synchronizing a mobile station with a base station

ABSTRACT

The invention relates to a method of synchronizing a mobile station with a base station in a wireless communications system, to a mobile station for a wireless communications system, as well as to a wireless communications system of this kind. In order to enable faster ultimate synchronization, it is proposed to compare components of the received data not only with a stored synchronization pattern, but also with a stored identification pattern prior to the adaptation of the timing of the mobile station to received data. Already before the synchronization it can thus be determined with a high degree of probability whether received data originates from a desired base station or not. The necessity of checking the contents of packets which are not associated with a desired base station can thus be avoided. Moreover, because of such preliminary checking, it is no longer necessary either to reset the timing of the mobile station after synchronization on the basis of data which later appear to originate from another base station.

The invention relates to a method of synchronizing a mobile station witha base station in a wireless communications system, to a mobile stationfor a wireless communications system, and to a wireless communicationssystem of this kind.

Wireless communications systems which include at least one base stationand at least one mobile station are known from practice, that is,notably for wireless communication in locally restricted areas. Anexample of a wireless communications system is the DECT (DigitalEnhanced Cordless Telecommunications) system which is described interalia in ETSI EN 300 175: “Digital Enhanced Cordless Telecommunications,Common Interface”.

On the one hand, 10 different frequencies are available for thetransmissions in a DECT system. On the other hand, the transmission timeis subdivided into frames, each of which comprises 24 time slots. Eachcombination of a given frequency and a given time slot constitutes aradio channel which can be used for transmissions by a base station or amobile station. In order to enable communication to be started between amobile station and a base station in a DECT system, it is necessary tosynchronize the mobile station first with the base station with which itis associated. To this end, in a first step the mobile station must findthe radio channel used by the relevant base station; in this respect itis to be noted that the mobile station can in most cases receive signalsfrom a plurality of base stations. The mobile station will also transmitat the frequency of said radio channel, be it offset by 12 time slots.The timing of the mobile station can then be adjusted on the basis ofthe signals received via said radio channel in a second step in such amanner that it corresponds to the timing of the base station. Thechanging of the time slots in the mobile station is thus tuned to thechanging of the time slots in the base station.

For the purpose of illustration FIG. 1 shows a typical radio environmentof a mobile station on the basis of a co-ordinate system. The radioenvironment is composed of the signals from various mobile stations andbase stations within the range of the mobile station considered. Thex-axis of the co-ordinate system is subdivided in conformity with theconfiguration of the time slots 0 to 23 used by the mobile stationconcerned prior to synchronization. The 10 different frequencies 0-9 ofthe system are plotted on the y-axis of the co-ordinate system. Darkrectangles in the diagram represent radio channels instantaneously usedby base stations and bright rectangles represent channelsinstantaneously used by mobile stations.

In the situation illustrated in the figure, transmissions take place byten different base stations, each station utilizing a different channel.Five of the transmissions by base stations as shown form part of aduplex connection which is associated each time as a respective secondpart with the mobile station transmissions which are also shown. Themobile stations transmit at the same frequency as the associated basestation, but 12 time slots after said base station. The channels shownare allocated only approximately a given time slot on the x axis,because the time slots used by the various stations usually do notcorrespond to the time slot of the relevant mobile station for which theco-ordinate system was conceived.

The mobile station under consideration itself does not use any channelyet for the transmission. For a synchronization so as to establishcommunication, the mobile station must search the radio channel of thebase station associated with the mobile station. It is assumed that inthis case this is the channel 11 with the carrier frequency 8 in thevicinity of the time slot 4 of the mobile station.

A procedure which is known from practice will now be described for thedetermination of this channel.

The mobile station first measures the received signal strength, notablya RSSI (received signal strength indicator) on all 240 channels inquestion. Subsequently, it activates its receiver successively for eachof the channels, that is, in the order of the signal strengths measuredand starting with the channel with the strongest signal. In theenvironment shown in FIG. 1 at least 15 channels exhibit a signalstrength that can be measured and must, therefore, be tested by themobile station. Because the time slots of the relevant station do notcorrespond to those of the searching mobile station, measurable signalstrengths may also be detected partly in two time slots of the searchingmobile station, so that the total number of channels to be tested maybecome larger than the number of actual transmissions.

The signals transmitted by a base station or a mobile station andreceived by the searching mobile station in the DECT system have theform of a DECT packet which is shown in FIG. 2, be it not to scale. Itis composed of a preamble 21, a synchronization word 22 of 16 bits, an Afield 23 and possibly a B field 24. When the searching mobile stationreceives a synchronization word 22, it can adapt its local timing tothat of the bit and time slot timing of the DECT packet, known on thebasis of the word, and hence to that of the transmitting station.

Therefore, for the synchronization the mobile station has to filterpackets of the base station associated with the mobile station from thesignals received. To this end, the mobile station utilizes a comparisoncircuit whereby packets of base stations can be recognized on the basisof the synchronization word 22. The synchronization word 22 has a valueof 0xE98A in packets of DECT base stations and a value of 0x1675 inpackets of DECT mobile stations.

FIG. 3 is a diagrammatic representation of such a comparison device. Itincludes a 16-bit shift register 31, a comparison circuit 32, and amemory 33 of the mobile station in which the value of the 16-bitsynchronization word 22 in packets of base stations is stored. Inputs ofthe comparison circuit 32 are connected to outputs of the shift register31 as well as to outputs of the memory 33. The input of the shiftregister 31 is also connected to the receiver of the mobile station (notshown) and the output of the comparison circuit 32 is connected to asynchronization device which is not shown either. The memory 33 need notform part of the comparison circuit itself; it may also merely beconnected to the comparison device.

The data bits 34 which are received at the instantaneous frequency bythe receiver of the mobile station are shifted through the 16-bit shiftregister 31. After each bit 34 newly shifted in, all 16 bits present inthe shift register 31 at that instant are transferred to the comparisoncircuit 32. The comparison circuit 32 also receives the 16-bitsynchronization word stored in the memory 33. The respective 16 bitsreceived are then compared in the comparison circuit 32. When the twoentries correspond, an output signal 35 which indicates that asynchronization word of a base station has been found is generated.

In order to compensate the deviations in the time slot timing, thereceiver and the comparison device of the mobile station are activatedfor a duration of two time slots for each channel. It is thus ensuredthat each synchronization word transmitted by a base station isrecognized by the comparison device in the mobile station.

FIG. 4 illustrates the detection of a synchronization word of a basestation over two time slots. In the Figure the activation and thedeactivation of the receiver and the comparison device of the mobilestation and a received DECT packet 41 are associated with the time slotsof the mobile station.

At the beginning of a first time slot N-1 both the receiver and thecomparison device of the mobile station are activated. The receiverreceives a DECT packet 41 at the adjusted frequency, which packetcommences inside the first time slot N−1 and also extends into thesecond time slot N because of the different timing of the transmittingstation and the mobile station in question.

If no synchronization word of a base station is recognized within twocomplete time slots N−1, N of the mobile station, because either nopacket is received at all in these time slots or only a packet ofanother mobile station, the receiver and the comparison device aredeactivated again at the end of the second time slot N. Subsequently,the further search takes place in the two time slots in which thechannel with the next-higher measured signal strength may be situated.

However, as soon as data bits are received for which a comparison withthe stored synchronization word by the comparison device indicates thatthe synchronization word associated with the base stations is concerned,the comparison device is deactivated again. Subsequently, the receiveris deactivated, after complete reception of the DECT packet 41 withwhich the synchronization word received is associated, in conformitywith the timing of the synchronization word and hence of thetransmitting base station; this means that deactivation does not have towait for the end of the second time slot N. The deactivation of thereceiver brings forward the beginning of the next time slot N+1, so thatthe local timing of the mobile station is adapted to that of the basestation.

Granted, the timing of the mobile station is now adapted to the timingof the base station wherefrom the received DECT packet 41 originates.However, the wrong base station may still be involved, so that prior tothe start of communication an identity of the transmitting base station,contained in the A field of the packet 41 received, must be tested byappropriate software of the mobile station.

When the software test reveals that a packet 41 of the base stationwhich is associated with the mobile station is concerned, the mobilestation retains the adjusted synchronization and the communication maycommence.

However, if the software test reveals that the packet 41 received is apacket from another base station, the previous timing of the mobilestation is restored and the search for a packet with a synchronizationword from a base station commences anew. The execution of such a newsearch is shown in FIG. 5 in which the activation and deactivation ofthe receiver and the comparison of the mobile station as well as thereceived DECT packet 41 are again associated with the time slots of themobile station. The further search in this case does not commence withthe next time slot N of the list. The receiver instead is activated inthe same time slot N−1 as in the preceding search. The comparisondevice, however, is activated only at an instant in the time slot N−1 asfrom which the already detected synchronization word from the wrong basestation cannot be detected again. Thus, only synchronization words whichappear later than the initially recognized synchronization word in thistime slot are found. Such repeated testing of the same time slot isnecessary because not all-base stations transmit a B field in the DECTpacket 41, so that the packets may be significantly shorter than a timeslot. For example, up to 9 packets may occur in a period of two timeslots.

Each of the described steps requires time and hence increases the totalamount of time elapsing before the mobile station is synchronized withthe correct base station. In case the searched base station with thechannel 11, determined by the frequency 8 and the time slot 4, is testedas the last one in the situation shown in FIG. 1, overall at least tenbase stations must be checked. For the first nine checks a subsequentreset of the mobile station to the original timing is required, followedeach time first by a renewed attempt in the same time slot. Moreover,five signals from mobile stations are checked. For these signals,however, the correct synchronization word is not found already in thecomparison device, so that as a result only the time necessary to checkwhether the correct synchronization word is present in the data receivedis added.

The time required before the ultimate synchronization is achieved issubstantially increased in the case of a large number of transmittingstations, because every undesirable signal to be checked requires anadditional amount of time. Thus, it is a drawback of the known method ofsynchronizing a mobile station with a base station in a wirelesscommunications system that the synchronization may require acomparatively long period of time.

Moreover, the synchronization word has a length of only 16 bits in DECTsystems. The probability of finding this word in noise or in thetransmitted data from base stations or mobile stations, therefore, isnot insignificant, so that the time required for the synchronization isincreased further.

It is an object of the invention to provide a method, a mobile stationand a wireless communications system which enable faster synchronizationof a mobile station with a base station in a wireless communicationssystem.

On the one hand, the object is achieved by means of a method ofsynchronizing a mobile station with a base station in a wirelesscommunications system, which method includes the following steps:

-   -   transmission of messages by the base station via a given channel        which is defined by the frequency and the time slot, at least        part of the messages containing a synchronization component and        an identification component which identifies the base station,    -   reception of one of the messages by the mobile station,    -   comparison of components of the received message with a stored        synchronization pattern and comparison of at least one component        of the received message with a stored identification pattern,        and    -   synchronization of the timing of the mobile station with the        timing underlying the received message if both comparisons have        yielded correspondence.

On the other hand, the object is also achieved by means of a mobilestation for a wireless communications system. The mobile stationcomprises first of all storage means for storing a synchronizationpattern and an identification pattern which identifies a given basestation of the wireless communications system. It also includes areceiving device for receiving messages at a selected frequency. Themobile station also includes a comparison device which is connected tothe storage means and the receiving device in order to comparecomponents of a received message with a synchronization pattern which isstored in the storage means and to compare at least one component of areceived message with an identification pattern which is stored in thestorage means. The mobile station also includes a synchronization devicewhich is connected to the comparison device in order to synchronize themobile station with a synchronization component of a received message ifthe comparison device has detected correspondence between components ofthe message and a stored synchronization pattern and a storedidentification pattern.

Finally, the object is also achieved in accordance with the invention bymeans of a wireless communications system. In addition to at least onemobile station in accordance with the invention, said communicationssystem also includes at least one base station which includes atransmission device for the transmission of messages, at least some ofwhich include a synchronization component and an identificationcomponent which identifies the base station.

The invention is based on the idea that the search for a base station bya mobile station in a wireless communications system can besignificantly accelerated when the identity of the mobile stationtransmitting a message is no longer checked after a first tentativesynchronization by a software implementation. Therefore, in accordancewith the invention the identity of the base station transmitting amessage is checked in the same processing step as the acquisition of avalid synchronization component in a message received, that is, prior tothe actual synchronization of the mobile station. The invention thusavoids incorrect synchronizations and, with every incorrectsynchronization avoided, the necessity of resetting the timing of themobile station and of searching for suitable synchronization componentsin received data in the same time slot again is also avoided. Moreover,in conformity with the invention it is no longer necessary to check, byway of a software implementation and after the synchronization, thecontents of data packets which do not belong to a desired base station,so that repeated checking of time slots is also avoided in the casewhere a plurality of packets occurs within a single time slot. Overall,the invention thus enables a reduction of the time required forsynchronization.

Granted, for example, in DECT systems each transmitted packet does notcontain an identification component. From the cited ETSI document,however, it appears that an identification is contained with a minimumprobability of 6.25% in a packet transmitted by a base station, that is,when the base station must transmit paging information and signalizationin each frame, and with a maximum probability of 93.75%, that is, whenthe base station does not have to transmit paging information andsignalization. A statistical evaluation of DECT packets in a Philipssemiconductor DECT system has revealed that approximately 89% of allpackets transmitted by a base station are identity messages. Suchmessages present can thus be evaluated already prior to asynchronization, thus reducing the overall time required on average forsynchronization.

A further advantage of the invention resides in the fact that when asynchronization component as well as an identification component arecompared, a larger component of the message must correspond to storedpatterns than when merely a synchronization component is compared. Theprobability of the pattern stored being present by chance in noise or indata from an arbitrary message is thus reduced. The probability that a16-bit synchronization word is discovered in noise then is once in65,536 tested bits and hence once in 56,888 ms of continuous testing.Because the search requires at least 100 ms in all time slots, this is anegligibly small effect. However, in the case of an additionally testedidentification component of 16 bits, the probability of discovering thecombination of the synchronization word and the identification componentin the noise is only once in 4294967296 bits and hence once in 62minutes and 8 seconds of uninterrupted testing.

At the same time the testing of a synchronization component as well asan identification component suffices, with a high degree of probability,for the identification of the correct base station. Assuming a tested16-bit identification component, on average a base station will have thesame 16 bits in the identification component as one in 65536 other basestations. Because the range of the base station is small, that is, a fewhundred meters, the probability of encountering two base stations withthe same tested identification component in the same geographicalenvironment, therefore, is negligibly small, even though of course stepsmust also be taken for this case.

Advantageous embodiments of the invention are disclosed in the dependentclaims.

The features of the claims 2 to 9 ensure that, in addition to thetesting of the identity of a transmitting base station in accordancewith the invention, conventional (notably software-based) testing of theidentity of a transmitting base station is also possible. This isimportant in the case where the mobile station is not yet associatedwith a given base station or the stored identity component istransmitted only infrequently by the base station. It is thus ensured,preferably by way of a switching-over facility, that a channel can befound also in such cases. The synchronization can then take placeexclusively on the basis of correspondence between a signal componentand the stored synchronization pattern, and a further evaluation of thesignals received can subsequently take place in software of the mobilestation.

If the assignment of the synchronization component and theidentification component to one another in a message is defined, asimultaneous comparison with the stored synchronization pattern and thestored identification pattern could take place, for example, by way of acorrespondingly long shift register into which the bits of the messagereceived are sequentially entered and which is capable of applying allbits received to a comparison circuit in one operation. Preferably,however, in conformity with the features of the claims 3 and 8 twocomparison means are used for the comparison of the two patterns. Thisoffers the advantage that the confirmation of the correspondence of thesynchronization component is separately available for further use. Sucha separate confirmation is used to check given radio signals such asso-called “bit slicers”. In that case the comparison means may includenotably a shift register and a comparison circuit which compares therespective instantaneous contents of the shift register with the memorycontents. The result of the comparison means for the component that canbe detected ahead in time of the other component can then be applied ina delayed fashion, via a counter, to an AND element which also receivesthe result of the second comparison means directly. The output value ofthe AND element thus provides information as regards the correspondenceof the two components in a message.

The characterizing part of claim 4 offers the advantage that the checkedpart of an identification component of a message can be selected independence on the system, for example, by way of different lengths ofthe checked component. This is done notably by checking a header of afield in a message which indicates whether the field contains anidentification of the base station, and that further in the field acomponent of a given length is checked as from a given position.Correspondences in the checked identification component of differentbase stations can thus also be avoided. The identification component ofvariable length is also referred to as a PARK (Portable Access RightsKey) in DECT systems.

The characterizing part of claim 5 yields a preferred version of thesynchronization in conformity with the invention. While making onlyminor modifications in the customary method, this version enables thecorrect base station to be found faster in most cases than by means ofthe customary methods. The search for the correct channel in the orderof signal strength of the channels is then based on the assumption thatthe mobile station in most cases wishes to contact one of the basestations which are nearest in space. Even when an identificationcomponent is not included in each message, the correct channel can befound with a very high degree of probability by setting a given numberof repeats for the checking of each channel. The checking of two timeslots of the mobile station per channel then ensures that each messageis considered completely during a comparison.

The characterizing part of claim 6 yields an alternative, preferredversion of the synchronization in accordance with the invention. Forexample, in a DECT system most messages are transmitted only once oronly during a short period of time. Messages which are only occasionallytransmitted, therefore, normally will not result again in falsecorrespondence in the case of a second attempt. The only messages whichare regularly transmitted are Qt messages, that is, except foridentification messages, are Qt messages. These messages, representingsystem messages, however, have a frequency of less than 1/30 of theidentification messages when two different Qt types are used. When fourQt types are used, the ratio is reduced even to 1/60. Therefore, falsecorrespondences will be recognized only in isolated cases. Theprobability of recognition of a false correspondence twice insuccession, therefore, is negligibly small. This effect can again becompletely avoided by utilizing the proposed PARK recognition.

The second preferred version, therefore, enables a significantacceleration to be achieved. Even when the base station only rarelytransmits an identification, which is not customary, the speed willstill be satisfactory. Because the signal strength is not taken intoaccount, the detection will take place without influencing by othersystems.

A comparison device implemented on the basis of software is alsofeasible when the comparison in a comparison device is preferablycarried out on the basis of hardware.

These and other aspects of the invention will be described in detailhereinafter with reference to the embodiments and the drawings. Therein:

FIG. 1 shows a typical radio environment for a DECT mobile station,

FIG. 2 shows the format of a DECT packet,

FIG. 3 shows diagrammatically a customary comparison device,

FIG. 4 shows an example of successful detection of a synchronizationword,

FIG. 5 shows an example of a repeated attempt for the detection of asynchronization word,

FIG. 6 is a diagrammatic representation of a comparison device of anembodiment of a mobile station in accordance with the invention,

FIG. 7 shows the format of a DECT A field, and

FIG. 8 is a diagrammatic representation of an embodiment of a comparisonmeans of a comparison device for the detection of the identity of a basestation in conformity with the invention.

The FIGS. 1 to 5 have already been described in the preamble of thepresent description.

The embodiments to be described hereinafter are based on a DECT systemin accordance with the invention which includes a mobile station inaccordance with the invention which is provided with a receiving device,storage means, a comparison device and a synchronization device.

First the comparison device of the mobile station will be described indetail. This comparison device is shown diagrammatically in FIG. 6.

The comparison device includes first comparison means 61 and secondcomparison means 62. The input of the first comparison means 61 and theinput of the second comparison means 62 are connected in parallel to thereceiving device (not shown) of the mobile station. The output of thefirst comparison means 61 is connected, via a counter 63, to a firstinput of an AND-gate 64 and the output of the second comparison means 62is connected directly to a second input of the AND-gate 64. The outputof the AND-gate 64 in its turn is connected to the synchronizationdevice (not shown) of the mobile station.

Both comparison means receive the data bits 65 which are received by thereceiving device of the mobile station. When such data bits 65 originatefrom a DECT base station, they form part of packets which correspond tothe DECT packet shown in FIG. 2; the B field 24 may be present in someof the packets and absent in other packets. In the field provided forthis purpose the packets contain a synchronization word 22 and,moreover, in approximately 89% of the packets an identification word inthe A field 23, which identification word identifies the base stationwherefrom the packet originates.

When the mobile station wishes to start communication with a basestation, first the first comparison means 61 are activated. The databits 65 received are compared with a synchronization pattern which isstored in the mobile station and has the value 0xE98A and hence isidentical to the synchronization word of the DECT packets originatingfrom base stations. As soon as a sequence of data bits which correspondsto the stored synchronization pattern is recognized, the counter 63 isstarted and the subsequent bits 65, originating from the A field of thepacket, are applied to the second comparison means 62. When the counter63 reaches a predetermined value, the sequence of data bits 65 thenpresent in the second comparison means 62 is compared with anidentification pattern which is stored in the mobile station. Because ofthe presetting of the comparison point by the counter 63, the secondcomparison means 62 need perform only a single comparison.

The result of the comparison in the first comparison means 61 and theresult of the comparison in the second comparison means 62 are appliedto the AND-gate 64. The supply of the result of the comparison in thefirst comparison means 61, however, is delayed by the counter 63, sothat the two results for a packet simultaneously reach the AND-gate.When the instantaneous data bits 65 in the second comparison means 62correspond to the identification pattern, via the AND-gate 64 an outputvalue 66 can be output which indicates the correspondence of thesynchronization word as well as of the compared identification componentin the instantaneous packet with the stored patterns. This triggerssynchronization of the mobile station with the synchronization componentof the received packet by the synchronization device. Moreover, thesoftware of the mobile station is informed on the positive result. Whenthe instantaneous data bits 65 do not correspond to the identificationpattern, the AND-gate 64 outputs an output value 66 which indicates thatno double correspondence has been found. Subsequently, the firstcomparison means 61 resume the search for a synchronization word.

As an alternative for two separate comparison means 61, 62, a singlecomparison of a correspondingly longer bit sequence could take place. Acomparison split into two parts, however, offers the advantage that itutilizes the detection of synchronization words only for the control ofgiven radio signals. A separate signal which indicates exclusively thata synchronization word has been found, therefore, is required in anycase.

When the synchronization device receives the information that thecomparison was successful, it synchronizes the mobile station in knownmanner in conformity with the synchronization word. Subsequently, thepacket obtained is checked in software whereby, for example, the rarecases can be detected where both the synchronization word and thechecked identification component appeared in noise or in data fromanother base station. At that instant it can also be checked whether thepacket does not originate from the base station associated with themobile station, but from another base station with the sameidentification component. If no incorrect detection is found in thesoftware either, the adjusted synchronization is maintained and thecommunication between the mobile station and the base station maycommence. When incorrect detection is recognized, the timing of themobile station must be reset and a search must be undertaken once morefor a data packet from the correct base station. Such incorrectdetection, however, is so rare that it does not have a significanteffect on the mean synchronization time.

Instead of using exclusively a recognized synchronization word so as totrigger the synchronization and subsequently checking the identity ofthe base station transmitting the synchronization word in software, themobile station thus is synchronized and the received signal is checkedfurther only if the synchronization word is found and the definedcomponent of the identification word is contained in the A field of apacket received. This means that packets from incorrect base stationsare not checked and that the time usually spent on the checking andsubsequent resetting of the timing of the mobile station can be saved.

For example, the comparison means which are diagrammatically shown inFIG. 3 can be used as the first comparison means 61 for the comparisonof received data bits 65 with the stored synchronization pattern.

The second comparison means 62 for the comparison of received data bits65 with the stored identity pattern may also be constructed like thecomparison means shown in FIG. 3. However, the comparison means 62 arepreferably constructed to be such that they enable more flexibledetermination of the identity of the transmitting base station.

In order to illustrate such flexible comparison means, first theincorporation of the identity word in the A field 23 of a DECT packet asshown in FIG. 2 will be described. As is shown in FIG. 7, the A field iscomposed of a header 71 of 8 bits, a data component 72 of 40 bits and aCRC (Cyclic Redundancy Check) component 73 of 16 bits. The 48 bits ofthe header 71 and the data component 72 are protected by the 16-bit CRCcomponent 73. The first three bits in the header 71 indicate the type ofcontents of the data component 72. When an identity message isconcerned, the first three bits of the header 71 have a value “011” andthe 40 bits of the data component 72 contain an identification of thetransmitting base station.

The identification component of a received packet, checked in the secondcomparison means 62 of FIG. 6, may have an arbitrary length and besituated in an arbitrary position in the A field in the case of aflexible construction of the comparison means. The identificationcomponent in that case need not be coherent either.

FIG. 8 shows an advantageous embodiment of flexible comparison means 62of the comparison device of FIG. 6 for the comparison of the identitypattern with an identification component from the A field of a DECTpacket as shown in FIG. 7.

The comparison means include a shift register 81 for the intermediatestorage of 48 bits a₀-a₄₇ and a comparison circuit 82. Also provided arestorage means 83 in which 42 bits are stored. The storage means 83 mayform part of the comparison means or may be general storage means inwhich further data is additionally stored for other devices of themobile station, notably for the comparison means 61 for thesynchronization word. The input of the shift register 81 corresponds tothe input of the second comparison means 62 of FIG. 6 and hence isconnected to the receiving device (not shown) of the mobile station.Inputs of the comparison circuit 82 are connected to outputs of theshift register 81 as well as to outputs of the storage means 83. Theoutput of the comparison circuit 82 corresponds to the output of thesecond comparison means 62 in FIG. 6 and hence is connected to theAND-gate 64 (not shown) of FIG. 6.

After a synchronization word has been detected in a received DECT packetin the first comparison means 61 of FIG. 6, the first 48 bits of the Afield of the received packet are sequentially shifted into the secondcomparison means 62 of FIG. 6, and hence into the shift register 81 ofFIG. 8, until all bits of the header and the data component of the Afield have been stored in the shift register 81. The instant at whichall bits 65 of the A field will have been written is preset by thecounter 63 of FIG. 6. The first three bits a₀-a₂ written into the shiftregister 81 and the bits a₉ to a₄₇ are then applied in parallel to thecomparison circuit 82. Similarly, the 42 bits stored in the storagemeans 83 are also applied in parallel to the comparison circuit, thefirst three bits having the value “011” while the further bitsconstitute an identification of the desired base station which isidentical to the bits a₉ to a₄₇ in an A field of DECT packets with anidentification component of the desired base station.

The comparison circuit 82 of the second comparison means 62 thencompares on the one hand the first three bits a₀-a₂ in the header 71 ofthe A field, indicating whether the received data 65 concerns anidentity message, with the corresponding bits from the storage means 83.As opposed to the first three bits in a first section 84 in the shiftregister 81, the six bits a₃ to a₈ in the subsequent section 85 are notapplied to the comparison circuit 82 and hence are not checked. A thirdsection 86 contains a bit sequence of a given length of the datacomponent 72 of the A field; this length may be dependent notably on thetype of system in which the mobile station wishes to communicate. Thisvariable identity component is also known as PARK and is contained inthe bits a₉-a_(n), where n may be between 9 and 47. Because of thevariable length of PARK, the comparison circuit 82 must be capable ofchecking an identity component of variable length. A final section 87with the bits a_(n+1) to a₄₇ again is not checked, the boundary 88between the third section 86 and the fourth section 87 being differentfrom one system to another. In the example shown in FIG. 8, the bits a₄₅to a₄₇ are assigned to the fourth section 87 and hence are ignoredduring the comparison. If correspondence was found between the receivedand the stored bits in the sections 84 and 86, a signal 89 whichindicates such correspondence is output.

The CRC bits a₄₈ to a₆₃ need hardly be included in the identity check,because the probability of correspondence of all identity bits with thestored pattern while the CRC sum is wrong is small enough to avoid anysignificant effect on the result.

The checking of an identity component in received DECT packets alreadybefore synchronization, of course, makes sense only when the mobilestation already knows the identity of the base station it searches. Thisis not necessarily the case, for example, during a first presentation ofa mobile station or when the base station has the desired identity onlyas a secondary or tertiary identity; in the latter case the identity canalso be transmitted, be it only rarely. In such cases the conventionalsynchronization is still necessary, provisional synchronization alreadytaking place merely on the basis of a synchronization word found.Therefore, the software of the mobile station includes an option whichis capable of switching over, if necessary, between the conventionalmethod and the method in accordance with the invention.

Hereinafter a description will be given of two preferred possibilitiesfor the use of the comparison device of the embodiment shown in theFIGS. 6 and 8 in a mobile station for synchronization with a desiredbase station when the mobile station is present in the radio environmentshown in FIG. 1.

The mobile station must again be synchronized with the base stationwhich utilizes the channel 11 which results from the combination of thefrequency 8 and the time slot 4 of the mobile station.

In conformity with a first, preferred alternative the synchronization isrealized as in the case of the conventional method described withreference to the FIGS. 4 and 5, except for the fact that a preselectionof the channels is already carried out by the second comparison means 62of the comparison device of the mobile station.

Thus, first the signal strength in the form of the RSSI of all possiblechannels is measured in the mobile station. Starting from the channelwith the strongest RSSI, a check of packets received via the channels isthen performed in the comparison device of the mobile station, that is,in the order of the measured RSSI, as described with reference to theFIGS. 6 to 8. The receiving device and the comparison device are thenactivated each time for up to two time slots. After the correctsynchronization word and the correct identification component have beenfound in a received packet within two time slots, the receiver isdeactivated in conformity with the contents of the synchronization wordso that the timing of the mobile station is adapted to the timing of thebase station. If no correspondence with the stored patterns is foundwithin the two time slots, the next channel is checked. If, after asuccessful synchronization, it is detected in the software that theselected channel was not the correct channel after all, the same twotime slots are checked again; the comparison then commences at aninstant within the time slot such that the previously incorrectlydetermined packet cannot be detected again.

Each channel is checked three times in succession as to whether thedesired synchronization word and the desired identification componentare present therein, that is, for as long as the correct channel has notbeen found. Assuming a probability of approximately 89% that anidentification word is present in a data packet, the desired basestation will be recognized with a probability of almost 99.9% when threeattempts are made during the checking of the corresponding channel.

The advantage of this alternative resides in the fact that, because ofthe second comparison means, the mobile station generally will not besynchronized with a base station with which it is not associated. Thetime required for the checking of channels of other base stations aftersuccessful synchronization in other conventional methods is thus saved.Granted, all channels having an RSSI higher than that of the desiredbase station must still be checked prior to successful synchronizationas before. However, in the presence of more than one transmitting basestation in the radio environment of the mobile station, the overallamount of time lost is significantly smaller than in the case of theconventional method, because neither an identity check by the softwarenor a regular resetting of the timing of the mobile station is required.This alternative represents an extension of the customary method whichcan be simply realized and implemented with only a small amount of workand coding.

Because the first alternative is based on the RSSI of the receivedsignals, the number of base stations in the system influences thesynchronization time. For example, when the desired base station is notthe base station with the strongest RSSI, time will be required to checkfirst the channels having the stronger RSSI. In the case of threeattempts per channel, it may be assumed that a time of approximately 30ms will be required per channel.

Instead of each time two time slots per channel, in conformity with asecond preferred alternative the mobile station utilizes a widereceiving window over 25 time slots each time for all channels of acarrier frequency. To this end it is assumed that the respectiveadjusted frequency of the receiver of the mobile station remains stablefor at least 25 time slots and hence for 10,416 ms. Customarily this isthe case only for receivers which operate in a so-called closed loopmode. The window is positioned in such a manner that it covers all 24possible channels of a frequency. Once more synchronization is carriedout only when, using the comparison device described with reference tothe FIGS. 6 to 8, a DECT synchronization word of a base station has beenfound as well as the desired identification component in the subsequentA field of a received packet.

If no correspondence is found for a carrier frequency, the next carrierfrequency is checked.

When a packet with the correct synchronization word and the correctidentification component is found for a carrier frequency, the mobilestation awaits the next identity message on this channel and carries outa new check so as to make sure that the message indeed originated fromthe correct base station. If it is found that it was not the correctbase station and that the values correspond only by chance, the searchis continued with a further window which commences exactly at the end ofthe synchronization word of the packet of the wrong base station. Thiswindow, however, terminates at the same time slot position as theoriginal window; this means that it is shorter than 25 time slots. Thecontinuation of the search after a packet has unduly been considered tobe correct thus corresponds to the procedure according to the firstalternative in the case of incorrectly recognized correspondence, be itthat in this case 25 time slots are used instead of two.

In this case the order of the strengths of measured RSSI is not checked;a sequential check of the 10 frequencies available is performed instead.

The actual synchronization again takes place by deactivation of thereceiver at an instant dictated by the correct synchronization word.

The synchronization time required hence is dependent on the frequency atwhich the identification message searched is transmitted and on theprobability of recognition of incorrect synchronizations.

Calculations have shown that the second alternative with the comparisondevice in accordance with the invention can lead to a significantacceleration of the synchronization in typical circumstances. Even inextreme circumstances, in which the base stations transmit informationmessages only rarely (being an exceptional case), the performance isstill satisfactory. Because an identity component is also detected priorto the actual synchronization and the evaluation of RSSI is dispensedwith in accordance with the invention, the method in conformity with thesecond alternative is not subject to any significant influencing byother systems. The second alternative requires a more complex conversionthan the first alternative, but offers a further enhancement of theperformance.

The invention has been described for the use in a DECT system and forthe packet formats customarily used in DECT systems. However, it can beused equally well for other wireless communications systems and forother packet formats.

1. A method of synchronizing a mobile station with a base station in awireless communications system, which method includes the followingsteps: transmission of messages by the base station via a given channelwhich is defined by the frequency and the time slot, at least part ofthe messages containing a synchronization component and anidentification component which identifies the base station, reception ofone of the messages by the mobile station, comparison of thesynchronization component of the received message with a storedsynchronization pattern and comparison of the identification componentof the received message with a stored identification pattern, andsynchronization of the mobile station with the timing underlying thereceived message if both two comparisons have yielded correspondence,wherein the mobile station carries out the comparisons in a time windowwhich comprises all relevant time slots, the comparisons aresequentially carried out in the time window for all relevant frequenciesfor as long as no correspondence is found for a frequency, whencorrespondence is found, new comparisons with components of a subsequentmessage are carried out for the same frequency and in the same positionof the time window, a synchronization of the mobile station inconformity with the synchronization component of the message takes placewhen the new comparisons reveal correspondence again, and when therepeated comparisons do not demonstrate correspondence, a window forfurther comparisons is positioned in such a manner that it commencesdirectly after the beginning of the synchronization component of thefirst message compared in the same position and that it ends at the endof the time window used thus far.
 2. A system that includes a basestation, and further includes a mobile station comprising storage means,a receiving device, a comparison device and a synchronization device,said system being configured for performing the method as claimed inclaim 1, and for selectively preventing, for a given one of saidmessages received, taking into account similarity between said onemessage and a stored identification pattern that would otherwise beassessed if not for the selection, in deciding whether the timing of themobile station is to be synchronized with the timing underlying said onemessage.
 3. A method as claimed in claim 1, characterized in thatcomponents of the received message are compared, separately from oneanother, with the stored synchronization pattern and with the storedidentification pattern, the results of the individual comparisons beingcombined so as to form an overall result for the synchronization.
 4. Amethod as claimed in claim 1, further comprising: storing, in the mobilestation, the identification pattern, to form said stored identificationpattern, and selecting, for the comparison with said at least onecomponent, a part of said stored identification pattern.
 5. A method asclaimed in claim 1, characterized in that first the strength of receivedsignals is measured separately for all relevant channels, thecomparisons for each channel being carried out in the order of thesignal strengths associated with the channels, starting with the channelhaving the strongest signal and each time with a predetermined number ofattempts, until correspondence is found for message components with thestored synchronization pattern as well as with the stored identificationpattern, the comparisons for each channel being based on the signalsreceived within two time slots of the mobile station.
 6. The use of amethod as claimed in claim 1 in a DECT (Digitally Enhanced CordlessTelecommunications) system.
 7. The method of claim 1, furthercomprising: forming, from said both two comparisons, a combined outputvalue; and performing, subject to the formed output value, saidsynchronization of the timing of the mobile station with the timingunderlying the received message.
 8. A mobile station for a wirelesscommunications system, which station includes storage means for storinga synchronization pattern and an identification pattern which identifiesa given base station of the wireless communications system, a receivingdevice for receiving messages at a selected frequency, a comparisondevice which is connected to the storage means and the receiving devicein order to compare a synchronization component of a received messagewith a synchronization pattern which is stored in the storage means andto compare an identification component of a received message with anidentification pattern which is stored in the storage means, wherein theidentification component identifies a base station, and asynchronization device which is connected to the comparison device inorder to synchronize the mobile station with a synchronization componentof a received message when the comparison device has detectedcorrespondence between the synchronization and identification componentsof the message and a stored synchronization pattern and a storedidentification pattern, respectively; characterized in that thecomparison device includes separate comparison means for the comparisonof components of a received message with a stored synchronizationpattern and for the comparison of at least one component of a receivedmessage with a stored identification pattern, each of the comparisonmeans being arranged to output an output value which indicates whethercorrespondence with the respective pattern has been detected, the mobilestation being provided with logic means which are arranged to combinethe output values of the two comparison means for each message so as toform a combined output value which indicates whether correspondence hasbeen detected with the stored synchronization pattern as well as withthe stored identification pattern.
 9. A mobile station as claimed inclaim 8, characterized in that it includes switching means which arearranged to supply the synchronization device either with the combinedoutput value of the two comparison means or only with the output valueof the comparison means for the comparison of components of the receivedmessage with a stored synchronization pattern, the synchronizationdevice being suitable to determine for both cases, on the basis of theoutput value received, whether the message is to be used forsynchronization of the mobile station.
 10. A wireless communicationssystem which includes at least one base station and at least one mobilestation, the base station including a transmission device for thetransmission of messages, at least part of which includes asynchronization component and an identification component whichidentifies the base station, the mobile station including storage meansfor storing a synchronization pattern and an identification patternwhich identifies a given base station of the wireless communicationssystem, a receiving device for receiving messages at a selectedfrequency, a comparison device which is connected to the storage meansand the receiving device in order to compare the synchronizationcomponent of a received message with a synchronization pattern which isstored in the storage means and to compare the identification componentof a received message with an identification pattern which is stored inthe storage means, and a synchronization device which is connected tothe comparison device in order to synchronize the mobile station with asynchronization component of the received message when the comparisondevice has detected correspondence between the synchronization andidentification components of the message and the stored synchronizationpattern and the stored identification pattern, respectively wherein thecomparison device includes separate comparison means for the comparisonof components of a received message with a stored synchronizationpattern and for the comparison of at least one component of a receivedmessage with a stored identification pattern, each of the comparisonmeans being arranged to output an output value which indicates whethercorrespondence with the respective pattern has been detected, the mobilestation being provided with logic means which are arranged to combinethe output values of the two comparison means for each message so as toform a combined output value which indicates whether correspondence hasbeen detected with the stored synchronization pattern as well as withthe stored identification pattern.
 11. A wireless communications systemas claimed in claim 10, characterized in that it is a DECT (DigitallyEnhanced Cordless Telecommunications) system.
 12. The wirelesscommunication system of claim 10, configured such that the detectingcorrespondence comprises forming, from the two comparisons, an outputvalue, and further configured for performing, subject to the formedoutput value, the synchronizing of the mobile station with saidsynchronization component of the received message.